Actually, that's one of my biggest problems when it comes to productivity. The REPL (read-eval-print loop) takes too long because the unit tests take too long to run, or the environment takes too long to load.

Recently, I took a day to tackle this problem for myself, and I've found that I'm just much more productive when I'm stuck in a quicker REPL loop, rather than when I need to wait and stare out the window--and then I get distracted.